Cell for the electrowinning of aluminium operating with metal-based anodes

ABSTRACT

A cell for the electrowinning of aluminium comprises a horizontal carbon cathode bottom ( 11 ) having an aluminium-wettable surface coating ( 25 ) and a series of plates ( 21 ) made of aluminium-wettable reticulated porous material, typically foams, filled with aluminium and placed flat on the aluminium-wettable surface coating ( 25 ). During use, a thin bottom layer of aluminium ( 22 ) wets the aluminium-wettable surface coating ( 25 ) on top of the cathode bottom ( 11 ) and a bottom part of the porous aluminium-filled plates ( 21 ). A top layer of aluminium ( 23 ) is formed above the porous aluminium-filled plates ( 21 ). The cell may be operated with metal anodes ( 10 ) possibly protected with a cerium oxyfluoride coating when operated above about 910°-930° C.

FIELD OF THE INVENTION

[0001] The invention relates to a cell for the electrowinning of aluminium from alumina dissolved in a crustless fluoride-containing molten electrolyte at a temperature below 930° C., as well as the production of aluminium in such cell.

BACKGROUND OF THE INVENTION

[0002] The production of aluminium today utilises cells for the electrolysis of alumina dissolved in cryolite with an excess of approximately 10 weight % aluminium fluoride, operating at a temperature of approximately 950° C., utilising carbon anodes.

[0003] Several patents have been filed and many granted concerning anode and cathode materials, shape, cell designs, operating conditions etc., and many solutions to specific problems have been proposed. However, no overall arrangement has heretofore been proposed which meets up to all the practical requirements for the industrial production of aluminium with low contamination.

[0004] Most metal anodes suggested until now, except anodes covered with a protective cerium-based coating, are highly soluble in the electrolyte utilised contaminating the aluminium produced, and have other drawbacks such as low electrical conductivity, short life and high cost.

[0005] U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian), U.S. Pat. No. 4,966,674 (Bannochie/Sheriff), U.S. Pat. Nos. 4,683,037 and 4,680,094 (both in the name of Duruz) describe metal anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained by the addition of small amounts of cerium to the molten cryolite.

[0006] EP Patent application 0 306 100 and U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all in the name of Nyguen/Lazouni/Doan) disclose aluminium production anodes having an alloy substrate protected with an oxygen barrier layer that is covered with a copper-nickel layer for anchoring a cerium oxyfluoride operative surface coating.

[0007] Several improvements of the cathodic side of aluminium production cells have been disclosed in the following patents.

[0008] WO01/42168 (de Nora/Duruz) and WO01/42531 (Nguyen/Duruz/de Nora) describe a carbon-containing component of a cell for the production of aluminium by the electrolysis of alumina dissolved in a cryolite-based molten electrolyte, which cell component is protected from attack by liquid and/or gaseous components of the electrolyte or products, such as aluminium, produced during cell operation by a slurry-applied aluminium-wettable coating. PCT publications WO96/07773 (de Nora) and WO98/53120 (Berclaz/de Nora) disclose cells for the production of aluminium having a horizontal cathode bottom covered with a slurry-applied aluminium-wettable coating.

[0009] It has also been proposed to stabilise the cathodic aluminium pool of conventional aluminium production cells by placing bodies onto the cathode bottom U.S. Pat. Nos. 5,472,578 and 5,865,981 (both in the name of de Nora) disclose a cell for the production of aluminium containing grids made of side-by-side upright or inclined walls whose bottom ends stand on a ceramic-coated carbon cell bottom covered by the pool of molten aluminium. Each grid has generally vertical through-openings dimensioned to allow the molten cell content to occupy the inside of the through-openings.

[0010] U.S. Pat. No. 4,600,481, (Sane/Wheeler/Gagescu/Debely/Adorian/Derivaz) and U.S. Pat. No. 4,650,552 (de Nora/Gauger/Fresnel/Adorian/Duruz) describe aluminium-wettable composite materials for use in contact with molten aluminium in an aluminium production cell. The composite materials are made of alumina and aluminium in particular with TiB₂. Slabs of this material may be used to cover a carbon cathode bottom of a conventional aluminium production cell.

OBJECTS OF THE INVENTION

[0011] One object of the invention is to provide an aluminium electrowinning cell incorporating metal-based anodes that can be operated without excessive contamination of the produced aluminium.

[0012] Another object of the invention is to provide an aluminium electrowinning cell that can achieve high productivity, low contamination of the product aluminium, and whose components resist corrosion and wear.

[0013] Yet another object of the invention is to provide an aluminium electrowinning cell including metal-based anodes which remain substantially insoluble under the cell operating conditions.

[0014] An overall object of the invention is to provide a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte which overcomes the various drawbacks of the previous proposals.

SUMMARY OF THE INVENTION

[0015] The invention proposes a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. This invention can be implemented in a conventional cell or can be applied to cells of new design.

[0016] The cell of the invention comprises a horizontal carbon cathode bottom having an aluminium-wettable surface coating and a series of plates made of aluminium-wettable reticulated porous material, typically foams, filled with aluminium and placed flat on the aluminium-wettable surface coating.

[0017] During use, a thin bottom layer of aluminium wets the aluminium-wettable surface coating on top of the cathode bottom, usually the entire or substantially the entire surface coating, and a bottom part of the porous aluminium-filled plates.

[0018] The application of an aluminium-wettable coating on the carbon cathode bottom underneath the aluminium-wettable reticulated porous plates leads to a singular improvement over prior art configurations.

[0019] Indeed, as opposed to the configuration disclosed in U.S. Pat. Nos. 4,600,481 and 4,650,552 mentioned above in which the aluminium-wetted ceramic foams rest directly on the aluminium-repellent carbon bottom of the cell, in the cell according to the present invention the aluminium-wettable porous plates rest on an aluminium-wettable coating which during use leads the aluminium to form a layer between the aluminium-filled porous plates and the aluminium-wettable coating. This aluminium layer covers and wets substantially the entire surface of the aluminium-wettable coating and wets also the bottom part of the aluminium-filled porous plates, whereby a continuous and substantially improved electrical contact is formed between the cathode bottom and the above located cathodic aluminium.

[0020] During use, a top layer of aluminium which is formed above the porous aluminium-filled plates and is covered with the electrolyte, provides an active cathode surface on which aluminium is cathodically reduced.

[0021] In a main embodiment of the invention, the cell comprises a series of metal-based anodes located above and parallel to the surface of the top layer of aluminium. Especially for cell operation above about 910°-930° C., each anode can have a metal-based anode substrate protected with an electrochemically active coating made of one or more cerium compounds that is maintained by the presence of cerium species in the electrolyte, as disclosed in U.S. Pat. Nos. 4,614,569, 4,966,674, 4,683,037, 4,680,094, 5,069,771, 4,960,494 and 4,956,068 mentioned above, and that prevents unacceptable contamination of the product aluminium by anode materials.

[0022] Other suitable metal-based anode materials optionally coated with the above cerium-based coating, include iron and nickel based alloys which may be heat-treated in an oxidising atmosphere as disclosed in WO00/06802, WO00/06803 (both in the name of Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), WO01/42535 (Duruz/de Nora), WO01/42534 (de Nora/Duruz) and WO01/42536 (Duruz/Nguyen/de Nora). Further oxygen-evolving anode materials are disclosed in WO99/36593, WO99/36594, WO00/06801, WO00/06805, WO00/40783 (all in the name of de Nora/Duruz), WO00/06800 (Duruz/de Nora), WO99/36591 and WO99/36592 (both in the name of de Nora).

[0023] Alternatively, the anodes can be consumable carbon anodes on which, during operation, CO₂ is formed.

[0024] The anodes may be spaced above the surface of the top layer of aluminium by a reduced anode-cathode distance (ACD) in the range of 20 to 40 mm. Such a reduced ACD permits cell operation with an increased electrolysis current density of about 0.6 to 1.2 A/cm² at the surface of the anodes. The increased current density produces sufficient heat to maintain cell stability while producing more aluminium.

[0025] Usually, the bottom layer of a aluminium a thickness in the range of 0.5 to 10 mm. The top layer of aluminium may have a thickness in the range of 5 to 100 mm and can even form a pool.

[0026] Preferably, the porous aluminium-filled plates have a thickness in the range of 10 to 100 mm.

[0027] The plates can be made of the materials disclosed in the aforementioned U.S. Pat. Nos. 4,600,481 and 4,650,552. Preferably, the plates are made of a reticulated ceramic material that is inert and resistant to molten aluminium having at its surface an aluminium-wetting agent, in particular a metal oxide that is reactable with molten aluminium as described below.

[0028] The inert and resistant ceramic material may comprise at least one oxide selected from oxides of aluminium, zirconium, tantalum, titanium, silicon, niobium, magnesium and calcium and mixtures thereof, as a simple oxide and/or in a mixed oxide, for example an aluminate of zinc (ZnAlO₄) or titanium (TiAlO₅). Other suitable inert and resistant ceramic materials can be selected amongst nitrides, carbides, borides and oxycompounds, such as aluminium nitride, AlON, SiAlON, boron nitride, silicon nitride, silicon carbide, aluminium borides, alkali earth metal zirconates and aluminiumates, and their mixtures.

[0029] The porous aluminium-filled plates preferably have a surface layer containing alumina, aluminium and a further metal, such as copper, iron and/or nickel. This surface layer is producible by exposing to molten aluminium the surface of aluminium-wettable plates which contains before use metal oxides, such as copper, iron and/or nickel oxides, that are reactable with molten aluminium. Other useful metal oxides that are suitable for reaction with molten aluminium are disclosed in WO01/42168 (de Nora/Duruz) and WO01/42531 (Nguyen/Duruz/de Nora).

[0030] Likewise, the aluminium-wetted surface coating on the carbon cathode preferably has a surface layer containing alumina, aluminium and a further metal, such as copper, iron and/or nickel. This surface layer is producible by exposing to molten aluminium an aluminium-wettable surface coating which contains before use metal oxides, such as copper, iron and/or nickel oxides, that are reactable with molten aluminium, as disclosed in the references mentioned above (WO01/42168 and WO01/42531). These references teach that in order to avoid contact between aluminium infiltrated into the aluminium-wetted coating and the substrate, a sub-layer of boride, for instance as disclosed in U.S. Pat. Nos. 5,364,513 and 5,651,874, which is inert and impervious to molten aluminium, or an aluminium-repellent layer can be used to anchor the coating onto the substrate.

[0031] In one embodiment, the coated metal structure of each anode has a horizontal expanse and is foraminate for guiding therethrough an electrolyte circulation from and to the electrochemically active coating. Suitable anode designs are disclosed in WO00/40781 and WO00/40782 (both in the name of de Nora).

[0032] The electrolyte may be at a temperature below 960° C., typically 8600 to 930° C. The electrolyte may comprises cryolite and, in addition to cryolite, an excess of AlF₃ in an amount of 15 to 30 weight % of the cryolite.

[0033] Electrolyte on the electrochemically active coating is preferably substantially saturated with alumina. Substantial alumina saturation can be achieved by using means for distributing alumina over a large area of the electrolyte, such as a plurality of alumina point feeders or a device for spraying alumina over the molten electrolyte, as disclosed in WO00/06804 (de Nora/Berclaz).

[0034] The cathode bottom may comprise a reservoir, for example located centrally in the cell, for collecting product aluminium. Also, The porous aluminium-filled plates may be arranged so that the top layer of aluminium located thereon drains into the reservoir.

[0035] The cell, in particular when it is retrofitted, may comprise a sideledge of frozen electrolyte and/or a crust of frozen electrolyte. However, the cell may also be operated with a crustless and ledgeless molten electrolyte, i.e. in an entirely molten state.

[0036] The invention also relates to a method of producing aluminium in a cell as described above. The method comprises feeding alumina to the electrolyte and passing an electrolysis current between the electrochemically active anode coatings and the top layer of aluminium to evolve gas, in particular oxygen, on the anodes and cathodically reduce aluminium.

[0037] A further aspect of the invention relates to a cell structure of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte. The structure comprises a horizontal carbon cathode bottom having an aluminium-wettable surface coating; and a series of plates made of aluminium-wettable reticulated porous material placed flat on the aluminium-wettable surface coating.

[0038] In a preferred embodiment, the cell structure comprises a series of metal-based anode substrates located above and parallel to the horizontal carbon bottom. Each anode substrate is protected with an electrochemically active coating made of one or more cerium compounds. Other metal-based anodes can also be used as mentioned above.

[0039] It is also possible to use consumable carbon anodes instead of cerium-based or other metal-based anodes. During operation of cells with carbon anodes, CO₂ is formed at the anodes surface instead of O₂

BRIEF DESCRIPTION OF DRAWINGS

[0040] The invention will be further described with reference to the accompanying schematic drawings, in which:

[0041]FIG. 1 is a cross-section through a drained cell of the invention with metal-based anodes; and

[0042]FIG. 2 is a cross-section through another drained cell of the invention with carbon anodes.

DETAILED DESCRIPTION

[0043] The cell shown in FIG. 1 has a horizontal carbon cathode bottom 11 whose surface is protected with an aluminium-wettable surface coating 25. The aluminium-wettable surface coating 25 is covered with a series of plates 21 made of aluminium-wettable reticulated porous material filled with aluminium. These plates 21 form a horizontal drained cathode surface 20 on which a top layer of aluminium 23 is produced, during use. A bottom layer of molten aluminium 22 wets substantially the entire aluminium-wettable surface coating 25 and a bottom part of plates 21.

[0044] The cathode bottom 11 comprises in the middle of the cell, a channel 30 for collecting product aluminium 60 drained from the adjacent aluminium-wettable cathode surfaces 20. The aluminium collection channel 30 is preferably coated with a slurry-applied refractory boride layer as described above.

[0045] The cell is fitted with metal-based anodes 10 on which during use oxygen is evolved.

[0046] The anodes 10 are resistant to the electrolyte 5 and to oxygen and other gases evolved during use, for example by being protected with a cerium oxyfluoride-based coating as disclosed in U.S. Pat. Nos. 4,614,569, 4,966,674, 4,683,037, 4,680,094, 5,069,771, 4,960,494 and 4,956,068 mentioned above. Alternatively, anodes 10 can be made of other suitable metal-based anode materials as mentioned above.

[0047] The cell comprises sidewalls 40, for example made of silicon carbide, which are covered with an aluminium-wetted wedge-shaped sidewall lining 41′ that extends from the periphery of the cathode bottom 11 to above the surface of the molten electrolyte 5 to shield the sidewalls 40 from molten electrolyte 5. The sidewall lining 41′ can be made of the same material as plates 21 and can be completely filled with molten aluminium retained in the material's pores by capillary effect.

[0048] To prevent the electrolyte 5 from freezing along the sidewall lining 41′ and on the surface of the electrolyte 5, the cell is thermally well insulated. As shown, the cell is fitted with an insulating cover 45 above the molten electrolyte 5. Details of suitable covers are disclosed in WO01/31086 (de Nora/Duruz).

[0049] To reduce the dissolution of the anodes 10 in the electrolyte, the cell may be operated with an electrolyte 5 at reduced temperature, typically from about 730° to 960° C., preferably from 860° to 930° C.

[0050] Operation with an electrolyte at reduced temperature reduces the solubility of oxides, including alumina. Therefore, it is advantageous to enhance alumina dissolution in the electrolyte 5.

[0051] Enhanced alumina dissolution may be achieved by utilising an alumina feed device which sprays and distributes alumina particles over a large area of the surface of the molten electrolyte 5. Suitable alumina feed devices are disclosed in greater detail in WO00/63464 (de Nora/Berclaz). Alternatively, alumina may be supplied by several conventional point feeders distributed of the molten electrolyte 5. Furthermore, the cell may comprise means (not shown) to promote circulation of the electrolyte 5 from and to the anode-cathode gap to enhance alumina dissolution in the electrolyte 5 and to maintain in permanence a high concentration of dissolved alumina close to the active surfaces of anodes 10, for example as disclosed in WO00/40781 (de Nora).

[0052] When the anodes 10 are protected with a cerium oxyfluoride-based coating, an amount of cerium species is preferably maintained in the electrolyte to maintain the coatings.

[0053] During operation of the cells shown in FIG. 1, alumina dissolved in the electrolyte is electrolysed to produce oxygen on the anodes 10 and aluminium at the cathodes that is incorporated into the top aluminium layer 23 on the drained cathode surfaces 20. Aluminium 60 from the top layer 23 drains into the collection channel 30 from where it can be tapped.

[0054]FIG. 2 where the same reference numerals are used to designate the same elements, illustrates a retrofitted cell utilising conventional consumable carbon anodes 10′ and operating with a frozen electrolyte crust 70 and ledge 71 that covers sidewalls 40, lining 41 and wedges 51.

[0055] In a variation, aluminium-wettable plates of larger size than shown in FIGS. 1 and 2 may be used, each larger plate extending over a significant part of a cathode block 11, in particular over the entire length across the cell of the a cathode block 11, preferably extending also over part of the channel 30 as disclosed in PCT/IB01/00953 (de Nora).

[0056] In a further variation, a retrofitted cell without an aluminium collection groove may operate with a top layer of aluminium that forms a cathodic aluminium shallow pool. Consequently, the inter-electrode distance may also be reduced which leads to a reduction of the cell voltage and energy savings. Furthermore, compared to conventional deep pool cells, a smaller amount of molten aluminium is needed to operate the cell which substantially reduces the costs involved with immobilising large aluminium inventories in aluminium production plants.

[0057] The production of aluminium-wettable reticulated porous material suitable to be used as a cathode plate for a cell of the invention will be further described in the following examples.

Example 1

[0058] An openly porous alumina structure (10 pores per inch which is equivalent to about 4 pores per centimetre) was rendered aluminium-wettable by coating it with two slurry-applied layers of different composition.

[0059] The first slurry of the first layer was made of 60 weight % particulate needle-shaped surface-oxidised TiB₂ (−325 mesh) having a TiO₂ surface oxide film, 3.3 weight % aluminium-wetting agent in the form of particulate Fe₂O₃ (−325 mesh) and 3.3 weight % TiO₂ powder (−325 mesh) in 33 weight % colloidal Al₂O₃ (NYACOL® Al-20, a milky liquid with a colloidal particle size of about 40 to 60 nanometer). When this slurry is heat treated, the colloidal alumina reacts with a TiO₂ surface oxide and the TiO₂ powder to form a mixed oxide matrix of Al₂O₃ and TiO₂ throughout the coating, this matrix containing and bonding the TiB₂ particles and the Fe₂O₃ particles.

[0060] The second slurry was made of 33 weight % of partly oxidised copper particles, 37 weight % of a first grade of colloidal alumina (NYACOL® Al-20) and 30 weight % of a second grade of colloidal alumina (CONDEA® 10/2 Sol, a clear, opalescent liquid with a colloidal particle size of about 10 to 30 nanometer).

[0061] An aluminium-wettable coating was applied onto the porous alumina structure by dipping this structure into the first slurry followed by drying for 4 hours at 40° C. and dipping it into the second slurry followed by drying for 15 hours are 40° C. The coated alumina structure was then heat treated for 3 hours in air at 700° C. to consolidate the coating.

[0062] The resulting structure is aluminium-wettable and is suitable to be wetted by aluminium before use or it can be wetted in-situ when used as a cathode plate for a cell of the invention.

[0063] The aluminium-wettable porous structure was wetted with alumina by dipping it in molten aluminium at 850° C. After 20 hours the wetted porous structure was extracted from the molten aluminium and allowed to cool down to room temperature.

[0064] Examination of the aluminium-wetted porous structure showed that it was completely filled with aluminium retained in the pores by the wettability of the structure and the capillary effect, and covered over the outer surface with aluminium.

[0065] The electrical resistivity of the aluminium-wetted structure was of the order of the resistivity of metal aluminium (2.65 μΩcm), whereas before wetting the structure had a resistivity of 35 to 45 kΩ.cm.

Example 2

[0066] An aluminium-wettable ceramic structure for use as a cathode plate in a cell according to the invention was made of a mixture of material inert and resistant to molten aluminium, i.e. alumina and titania, and aluminium-wettable material, i.e. copper oxide. The ceramic structure was prepared by coating a polyurethane foam with a slurry of ceramic particles followed by a heat treatment.

[0067] The slurry of ceramic material consisted of a suspension of 40 g particulate Al₂O₃ with an average particle size of 10 to 20 micron, 2.5 g of particulate CuO with a particle size of less than about 45 micron, 2.5 g of particulate TiO₂ with a particle size of less than about 45 micron in a colloidal alumina carrier consisting of 93 g deionised water and 6.6 g colloidal alumina particles with a colloidal particle size of about 10 to 30 nanometer.

[0068] A polyurethane foam having 10 to 20 pores per inch (equivalent to about 4 to 8 pores per centimetre) was dipped into the slurry and dried in air at 400 to 50° C. for 20 to 30 minutes. The dipping was repeated three times.

[0069] After dipping, the foam was dried in air at 50° C. for 4 to 5 hours. The foam contained about 0.3 to 0.5 g/cm³ of the dried slurry. The drying was followed by a heat treatment at about 8500 to 1000° C. in air for 4 to 5 hours to eliminate the polyurethane foam and consolidate the ceramic material formed from the slurry into a self-sustaining foam. This heat treatment was followed by an aluminisation treatment by immersion in molten aluminium for 2 hours in molten aluminium at 850° C.

[0070] The aluminised foam was extracted from the molten aluminium, allowed to cool to room temperature and cut perpendicular to a surface.

[0071] Examination of the aluminised foam showed that the polyurethane foam had disappeared. The TiO₂ had reacted with Al₂O₃ in the ceramic foam to form a titanium-aluminium mixed oxide matrix. CuO present at the surface of the ceramic foam had reacted with molten aluminium to produce an aluminium-wetted surface layer of Al₂O₃ and an alloy of copper and aluminium. The pores of the ceramic foam were completely filled with molten aluminium.

[0072] In a variation, the heat treatment step and the aluminisation step are carried out simultaneously as a single step. In a further variation, the copper oxide of the ceramic structure is replaced partly or completely with iron oxide and/or nickel oxide.

Example 3

[0073] An openly porous silicon carbide structure (30 pores per inch which is equivalent to about 12 pores per centimetre) for use as a cathode plate in a cell according to the invention was rendered aluminium-wettable by coating it with a slurry-applied layer.

[0074] The slurry consisted of 75 g surface oxidised iron particles (−325 mesh), 75 g Silica sol Nyacol 830 (a milky aqueous liquid containing 32 weight % colloidal silicon hydroxide that is converted into silica upon heat treatment) and 0.35 g of an aqueous solution containing 15% PVA (polyvinyl alcohol) that was used to adjust the viscosity of the slurry.

[0075] The openly porous structure was dipped onto the slurry and then dried for 30 min. at 60° C. The impregnated porous structure contained 0.278 g/cm³ of dried slurry including 0.214 g/cm³ surface oxidised iron particles.

[0076] The resulting structure was aluminium-wettable and suitable to be wetted by aluminium before use or in-situ when used as a cathode.

[0077] The aluminium-wettable porous structure was wetted with aluminium by dipping it in molten aluminium at 850° C. After 15 hours the wetted porous structure was extracted from the molten aluminium and allowed to cool down to room temperature.

[0078] Examination of the aluminium-wetted porous structure showed that it was filled with aluminium retained in the pores by the wettability of the structure and the capillary effect, and covered over the outer surface with aluminium. The pores had an aluminium filling ratio that was greater than 90 vol %.

[0079] The aluminium-wetted materials of Examples 1 to 3 can also be used to produce a sidewall lining or another cell component exposed to at least one of molten aluminium, molten electrolyte and oxidising or corrosive gas such as anodically produced oxygen. 

1. A cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, comprising: a horizontal carbon cathode bottom protected with an aluminium-wettable surface coating, the coated cathode bottom resisting corrosion and wear; a series of plates made of aluminium-wettable reticulated porous material filled with aluminium and placed flat on the aluminium-wettable surface coating; a thin bottom layer of aluminium between the porous aluminium-filled plates and the aluminium-wettable surface coating on top of the carbon cathode bottom which thin aluminium layer wets the surface coating and a bottom part of the porous aluminium-filled plates; and a top layer of aluminium which is formed above the porous aluminium-filled plates and covered with the electrolyte.
 2. The cell of claim 1, comprising a series of metal-based anodes located above and parallel to the surface of the top layer of aluminium.
 3. The cell of claim 2, wherein each metal-based anode has a metal-based substrate protected with an electrochemically active coating made of one or more cerium compounds that is maintained by the presence of cerium species in the electrolyte.
 4. The cell of claim 2 or 3, wherein the metal-based anodes are comprise an iron and nickel based alloy.
 5. The cell of any one of claims 1 to 4, wherein the anodes are spaced above the surface of the top layer of aluminium by a reduced anode-cathode distance in the range of 20 to 40 mm.
 6. The cell of any preceding claim, wherein the bottom layer of aluminium has a thickness in the range of 0.5 to 10 mm.
 7. The cell of any preceding claim, wherein the top layer of aluminium forms a pool.
 8. The cell of any preceding claim, wherein the top layer of aluminium has a thickness in the range of 5 to 100 mm.
 9. The cell of any preceding claim, wherein the porous aluminium-filled plates have a thickness in the range of 10 to 100 mm.
 10. The cell of any preceding claim, wherein the porous aluminium-filled plates have a surface layer containing alumina, aluminium and a further metal, such as copper, iron and/or nickel, the surface layer being producible by exposing to molten aluminium the surface of aluminium-wettable plates which contains before use metal oxides, such as copper, iron and/or nickel oxides, that are reactable with molten aluminium.
 11. The cell of any preceding claim, wherein the aluminium-wetted surface coating on the carbon cathode comprises a surface layer containing alumina, aluminium and a further metal, such as copper, iron and/or nickel, the surface layer being producible by exposing to molten aluminium an aluminium-wettable surface coating which contains before use metal oxides, such as copper, iron and/or nickel oxides, that are reactable with molten aluminium.
 12. The cell of any preceding claim, wherein the coated metal structure of each anode has a horizontal expanse and is foraminate for guiding therethrough an electrolyte circulation from and to the electrochemically active coating.
 13. The cell of any preceding claim, wherein the electrolyte is at a temperature below 960° C.
 14. The cell of claim 13, wherein the electrolyte is at a temperature in the range from 8600 to 930° C.
 15. The cell of any preceding claim, wherein the electrolyte comprises cryolite and, in addition to cryolite, an excess of AlF₃ in an amount of 15 to 30 weight % of the cryolite.
 16. The cell of any preceding claim, wherein the electrolyte on the electrochemically active coating is substantially saturated with alumina.
 17. The cell of any preceding claim, comprising means for distributing alumina over a large area of the electrolyte.
 18. The cell of any preceding claim, wherein the cathode bottom comprises a reservoir for collecting product aluminium.
 19. The cell of claim 18, wherein the porous aluminium-filled plates are arranged so that the top layer of aluminium located thereon can drain into the reservoir which is located centrally in the cell.
 20. The cell of any preceding claim, which comprises a sideledge of frozen electrolyte and/or a crust of frozen electrolyte.
 21. A method of producing aluminium in a cell as defined in any preceding claim, comprising feeding alumina to the electrolyte and passing an electrolysis current between the electrochemically active anode coatings and the top layer of aluminium to evolve gas, in particular oxygen, on the anodes and cathodically reduce aluminium.
 22. A cell structure of a cell for the electrowinning of aluminium from alumina dissolved in a fluoride-containing molten electrolyte, said structure comprising a horizontal carbon cathode bottom protected with an aluminium-wettable surface coating, the coated cathode bottom resisting corrosion and wear, and a series of plates made of aluminium-wettable reticulated porous material placed flat on the aluminium-wettable surface coating, the aluminium-wettable surface coating and the plates thereon being arranged for the formation therebetween of thin aluminium layer that wets the surface coating and a bottom part of the porous aluminium-filled plates during use. 